Somatostatin (SST) is a cyclic tetradecapeptide which was isolated for the first time from the hypothalamus as a substance which inhibits the growth hormone (Brazeau P. et al., Science 1973, 179, 77-79). It also operates as a neurotransmitter in the brain (Reisine T. et al., Neuroscience 1995, 67, 777-790; Reisine et al., Endocrinology 1995, 16, 427-442). The heterogeneity of the biological functions of somatostatin and the structure-activity relationships of its peptide analogues have led to the discovery of 5 sub-types of membrane receptors (Yamada et al., Proc. Natl. Acad. Sci. U.S.A, 89, 251-255, 1992; Raynor, K. et al, Mol. Pharmacol., 44, 385-392, 1993). Molecular cloning has made it possible to show that the bioactivity of somatostatin depends directly on these five sub-types of receptors.
The functional roles of these receptors are currently being actively studied. Preferential activation of sub-types 2 and 5 has been associated with the suppression, in the adenomas secreting these hormones, of the growth hormone GH (acromegalia), of TSH and prolactin; but the precise role of each sub-type remains to be determined